The present invention relates to a configuration for a spacecraft and methods for operating and controlling a spacecraft, particularly a spacecraft arranged in a highly inclined orbit.
Spacecraft often are placed in different orbits depending on their use. For example, intelligence gathering surveillance satellites typically have a low orbit altitude and are often launched into a circular near-polar orbit. This can increase their intelligence gathering utility by providing global coverage with twice daily passes over a particular geographic area. On the other hand, weather satellites may be placed into geosynchronous orbit to permit weather data to be continuously gathered in one location.
A new use of satellites is to broadcast audio as competition for terrestrial radio broadcasts. A common feature of Digital Audio Broadcast (DAB) systems is that they use a constellation of satellites in highly inclined orbits (HIO) to broadcast with high elevation angles to users in northern or southern latitudes. For example, the SIRIUS Satellite Radio system uses three satellites in 24 hour HIO orbits spaced 8 hours apart in repeating ground tracks. Each satellite operates in a window of xc2x14 hours about orbit apogee, denoted as the xe2x80x9capogee service arcxe2x80x9d. This system provides 100 channels of digital audio to mobile users in the United States with compatible DAB receivers. Another proposed system is Global Radio, which will use a constellation of three to six HIO satellites to provide DAB to mobile markets in Europe.
For DAB and other related applications, the advantage of the HIO orbit is that it can provide high elevation angles from the user to the satellite. Typical elevation angles range from about 70xc2x0 above the horizon to directly overhead (i.e., an elevation angle of 90xc2x0), over regions of high latitude, for example, from about 30xc2x0 to 60xc2x0 latitude. In contrast, for similar latitudes the standard equatorial geosynchronous (GEO) orbit can at best provide elevation angles of only about 55xc2x0 to about 36xc2x0.
The high elevation angles typically improve the quality of service by reducing signal blockages due to terrestrial obstructions. However, HIO systems have some drawbacks. One drawback of HIO systems is that costly modifications may be necessary to adapt a standard GEO spacecraft for operation in HIO. For example, the typical GEO spacecraft, an example of which is shown in FIG. 1, uses heat dissipating radiator panels that face in the north and south directions and flat-panel solar arrays that rotate about an axis, typically the pitch axis, that is roughly parallel to the earth pole axis. The efficiency of the radiators and the effectiveness of the solar arrays typically rely on the fact that the angle between the sun and the equatorial, or orbit, plane does not vary by more than about 23.5xc2x0. However, in a typical HIO orbit, which is inclined by about 40xc2x0 to about 65xc2x0, the sun angles with respect to the orbit plane can vary up to nearly about 90xc2x0 (65xc2x0 plus 23.5xc2x0), rendering the standard GEO spacecraft configuration unusable.
The present invention provides a method for controlling a spacecraft. The method includes orienting the spacecraft in an orbit inclined about 40 degrees to about 65 degrees. A yaw axis and a roll axis of the spacecraft and north and south radiator panels of the spacecraft are oriented parallel to the earth equatorial plane. A solar array rotation axis is oriented parallel to the earth pole axis. A coordinate frame of a spacecraft payload is rotated about a minus roll axis of the spacecraft at an angle equivalent to an orbit inclination of the spacecraft, thereby maintaining elements of a payload pointing toward a desired region on the earth. A spacecraft attitude is corrected to maintain the payload elements pointing at the desired region on the earth.
Additionally, the present invention concerns a method for assembling a spacecraft. The method includes mounting thermal radiator panels such that in use they will be oriented parallel to an equatorial plane of the earth to face in a north-south direction. Flat panel solar arrays are mounted to rotate about a single axis that in use will be parallel to an earth pole axis. Payload antennas are mounted such that in use the antennas are rotated in a coordinate frame of the spacecraft at an angle substantially similar to an orbit inclination angle of the spacecraft.
Additionally, the present invention provides a spacecraft operable in an orbit inclined about 40 degrees to about 65 degrees. The spacecraft includes flat panel solar arrays that rotate about a single axis parallel to the earth pole axis. The spacecraft also includes thermal radiators facing in a north-south direction, such that the thermal radiator panels are parallel to the equatorial plane. Furthermore, the spacecraft includes payload antennas mounted to the spacecraft such that the payload antennas are rotated in a spacecraft coordinate frame, wherein the rotation angle of the payload antennas is substantially similar to an inclination of the orbit of the spacecraft.
Furthermore, the present invention provides a spacecraft constellation that includes about 3 to about 6 spacecraft operable to transmit digital audio broadcasts. The spacecraft are arranged in orbits inclined about 40 degrees to about 65 degrees. The spacecraft include flat panel solar arrays that rotate about a single axis parallel to the earth pole axis. The spacecraft also include thermal radiators facing in a north-south direction such that the thermal radiator panels are parallel to the equatorial plane. Additionally, the spacecraft include payload antennas mounted to the spacecraft such that the payload antennas rotate in a spacecraft coordinate frame, wherein the rotation angle of the payload antennas is substantially similar to an inclination of the orbit of the spacecraft.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from a review of the following detailed description. The detailed description shows and describes preferred embodiments of the present invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the drawings and description are illustrative in nature and not restrictive.